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LTC3839 Datasheet, PDF (23/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller
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LTC3839
APPLICATIONS INFORMATION
higher temperature than required. Several capacitors may
also be paralleled to meet size or height requirements in
the design. Due to the high operating frequency of the
LTC3839, additional ceramic capacitors should also be
used in parallel for CIN close to the IC and power switches
to bypass the high frequency switching noises. Typically
multiple X5R or X7R ceramic capacitors are put in parallel
with either conductive-polymer or aluminum-electrolytic
types of bulk capacitors. Because of its low ESR, the
ceramic capacitors will take most of the RMS ripple cur-
rent. Vendors do not consistently specify the ripple current
rating for ceramics, but ceramics could also fail due to
excessive ripple current. Always consult the manufacturer
if there is any question.
Figure 6 represents a simplified circuit model for calculat-
ing the ripple currents in each of these capacitors. The
input inductance (LIN) between the input source and the
input of the converter will affect the ripple current through
the capacitors. A lower input inductance will result in less
ripple current through the input capacitors since more
ripple current will now be flowing out of the input source.
LIN
1μH
ESR(BULK)
+– VIN
ESL(BULK)
+
CIN(BULK)
ESR(CERAMIC)
ESL(CERAMIC)
CIN(CERAMIC)
IPULSE(PHASE1)
IPULSE(PHASE2)
3839 F06
Figure 6. Circuit Model for Input Capacitor
Ripple Current Simulation
For simulations with this model, look at the ripple current
during steady-state for the case where one phase is fully
loaded and the other was not loaded. This will in general
be the worst case for ripple current since the ripple cur-
rent from one phase will not be cancelled by ripple current
from the other phase.
Note that the bulk capacitor also has to be chosen for
RMS rating with ample margin beyond its RMS current
per simulation with the circuit model provided. For a lower
VIN range, a conductive-polymer type (such as Sanyo
OS-CON) can be used for its higher ripple current rating
and lower ESR. For a wide VIN range that also require
higher voltage rating, aluminum-electrolytic capacitors are
more attractive since it can provide a larger capacitance
for more damping. An aluminum-electrolytic capacitor
with a ripple current rating that is high enough to handle
all of the ripple current by itself will be very large. But
when in parallel with ceramics, an aluminum-electrolytic
capacitor will take a much smaller portion of the RMS
ripple current due to its high ESR. However, it is crucial
that the ripple current through the aluminum-electrolytic
capacitor should not exceed its rating since this will
produce significant heat, which will cause the electrolyte
inside the capacitor to dry over time and its capacitance
to go down and ESR to go up.
The benefit of PolyPhase operation is reduced RMS cur-
rents and therefore less power loss on the input capaci-
tors. Also, the input protection fuse resistance, battery
resistance, and PC board trace resistance losses are also
reduced due to the reduced peak currents in a PolyPhase
system. The details of a close form equation can be found
in Application Note 77 “High Efficiency, High Density,
PolyPhase Converters for High Current Applications”.
Figure 7 shows the input capacitor RMS ripple currents
normalized against the DC output currents with respect
to the duty cycle. This graph can be used to estimate the
maximum RMS capacitor current for a multiple-phase
application, assuming the channels are identical and their
phases are fully interleaved.
0.6
0.5
0.4
1-PHASE
2-PHASE
3-PHASE
0.3
4-PHASE
6-PHASE
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (VO/VIN)
3839 F07
Figure 7. Normalized RMS Input Ripple Current
3839fa
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